This invention relates to an electrical power convertor circuit.
Electrical power convertor circuits typically comprise a transformer with primary and secondary windings to match a load to a power source. Semiconductor switches control the flow of current from the power source into the primary windings. A control circuit modulates the semi-conductor switches, thereby controlling the current through the transformer primary. The primary and secondary windings are disposed in the transformer to effect desired coupling. An output rectifier and/or filter conditions the current flowing from the secondary to the load.
In order to improve operating efficiency and to achieve a higher operating frequency than is normally possible with "hard switching" or pulse-width modulated convertors, a series resonant configuration is employed, in which inductive properties of the transformer are made to resonate deliberately with series connected capacitive elements in the convertor circuit.
One type of series resonant convertor, used as a DC to DC convertor, is described in detail in the paper "Modelling The Full-Bridge Series-Resonant Convertor" by R. J. King and T. A. Stuart which appeared in the IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-18, No. 4, July 1982, pp. 449-459. A half-bridge version of this full-bridge convertor employing a centre-tapped resonant capacitor is described by FC Schwartz in "An Improved Method of Resonant Current Pulse Modulation for Power Convertors", IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IEI-23, No. 2, May 1976, pp 133-141.
A primary advantage of the series resonant convertor is that it permits higher switching frequencies than are normally possible with the commonly used pulse width modulated convertor. This makes it suited to aerospace applications, as higher switching frequencies facilitate the use of smaller transformers, inductors and capacitors, thereby reducing the volume and mass of the convertor.
It is possible to operate a series-resonant convertor at a rate either above or below the resonant frequency determined by the resonant frequency of the resonant inductor and capacitor. If the convertor circuit is operated below the resonant frequency, the current naturally goes to zero owing to the natural oscillation of the circuit, and turn-off of the circuit at the zero crossing point or shortly thereafter is completely stress-free and is known as zero current switching. In this mode of operation, while turn-off of the one semiconductor switch is relatively loss-free, turn-on of the next semiconductor switch results in significant losses due to the voltage and current appearing simultaneously across the switch at the time of turn-on. The addition of snubber capacitors across the semiconductor switches increases the severity of the turn-on losses. Even the internal capacitance of semiconductor switching devices such as MOSFETS can cause turn-on losses which are sufficient to restrict operation of the circuit.
If the series-resonant convertor circuit is operated above resonant frequency, the switches are turned off while current is still flowing through them. In this case, snubber capacitors can be shunted across the switches so as to reduce turn-off losses. The inductive current at turn-off charges and discharges these capacitors, and subsequently flows through freewheel diodes which may similarly be shunted across the switches, thereby equalising the voltage across the switches at turn-on, which results in zero voltage switching.
One version of a zero current switching convertor is described in the paper "Capacitor Voltage Clamp Series Resonant Power Supply With Improved Cross Regulation" by J. P. Agrawal et al, IEEE-IAS Annual Meeting of 1989, pp 1141 to 1146. In the circuit, the freewheel diodes are removed from across the controlled switches to a position in which they act as clamping diodes across a pair of capacitors making up the resonant circuit. This circuit has the advantage in that turn-on losses due to diode reverse recovery are eliminated. A concomitant disadvantage is that the resonant current peak increases in inverse proportion to the output voltage. Furthermore, the switching frequency decreases at reduced loading and output voltage. A solution to this problem has been proposed in the paper "Characteristics of a New Series-Resonant Convertor with a Parallel Resonant Circuit" by Kuwata et al, at the 1987 Intelec '87 Conference in Stockholm. A parallel-resonant circuit with a resonant frequency of over 20 kHz is installed in series with the load circuit. The output voltage is thus regulated by varying the conversion frequency in a higher area than that of a conventional series-resonant convertor. This circuit still suffers from significant turn-on losses.